"History calls those men the greatest who have ennobled themselves by working for the common good; experience acclaims as happiest the man who has made the greatest number of people happy." - Karl Marx, Reflections of a Young Man (1835)
You've probably heard opinions such as
"Ideas shape the course of history." - John Maynard Keynes
"You can kill a man, but you can't kill an idea." - Medgar Evars
These statements, if taken literally, embody a very old and common philosophical trend called idealism. This is the view that our ideas have a separate existence from our material world, and that these ideas are what actually shape us and our reality. Some idealists even go so far as to say that ideas are the only things that actually even exist, and material reality is either unknowable or entirely a matter of opinion.
This is a magical, unrealistic way of thinking. To believe in this sort of philosophy means ignoring what we actually experience, ignoring our knowledge of science in general and neurology, chemistry, and sociology, in particular.
In contrast to the idealists, Marxists believe that our ideas do not exist separately from us, and that knowledge comes not just directly and exclusively from our senses as the empiricists believed, but through our lived social experience, through our actual material conditions. Marx said,
"It is not the consciousness of men that determines their existence, but their social existence that determines their consciousness."
We receive data through our senses yes, but the form that data takes, the connections and sense we make of that data, is an ongoing process in our daily life, not just as isolated individuals but most importantly as productive social beings.
Our ideas, our worldviews, our philosophies and our culture, then, is rooted in our physically and socially determined situations. This is why the views of say rich bankers will be very very different from those of the homeless. This also explains why, typically, as someone's station in life changes, so also does their views.
Further, we inherit our situations in our lives from our parents, and along with that many of their views. We also receive many of our views from interacting with others who exist in a similar social situation as ourselves.
These physically and socially determined situations that we live in are what Marx called our material conditions. He took this view of a humanity shaped by material conditions and applied it to human history. This application is what Marx calls Historical Materialism, the theory that history is shaped, not by ideas, but by material conditions.
And what is the primary, though not the only, determining factor of our material conditions? How we relate to the means of production in our society. The means of production refers to the tools and materials we use to make society function, and the way we relate to the means of production is a question of power and control. Some members of society own the means of production, and others do not.
We call those who own and control the means of production the ruling class of our society. Those who neither own nor control these means are subject to those that do. All of human history is guided and defined by this struggle between the interests of the ruling classes and their subjects. The material forces of production shape the nature of the classes that struggle for control over them.
Armed with this clear, realistic understanding, history can be analyzed scientifically by examining the dominant mode of production in use within a society. One can even use this information to make accurate predictions about societal development.
So to summarize:
1. Ideas do not come from nowhere, they come from our material conditions.
2. Our material conditions are defined by the social situations in which we live.
3. Our social situations are determined mainly by how we relate to the means of production in our society.
4. All of history is governed by these principles and is defined by constant struggle for the control over the means of production.